In a troubling new discovery, scientists studying ocean waters off California, Oregon and Washington have found the first evidence that increasing acidity in the ocean is dissolving the shells of a key species of tiny sea creature at the base of the food chain.

The animals, a type of free-floating marine snail known as pteropods, are an important food source for salmon, herring, mackerel and other fish in the Pacific Ocean. Those fish are eaten not only by millions of people every year, but also by a wide variety of other sea creatures, from whales to dolphins to sea lions.

If the trend continues, climate change scientists say, it will imperil the ocean environment.

"These are alarm bells," said Nina Bednarsek, a scientist with the National Oceanic and Atmospheric Administration in Seattle who helped lead the research. "This study makes us understand that we have made an impact on the ocean environment to the extent where we can actually see the shells dissolving right now."

Scientists from NOAA and Oregon State University found that in waters near the West Coast shoreline, 53 percent of the tiny floating snails had shells that were severely dissolving -- double the estimate from 200 years ago.

Until now, the impact on marine species from increasing ocean acidity because of climate change has been something that was tested in tanks in labs, but which was not considered an immediate concern such as forest fires and droughts.

The new study, published in the Proceedings of the Royal Society B, a scientific journal based in England, changes that.

"The pteropods are like the canary in the coal mine. If this is affecting them, it is affecting everything in the ocean at some level," said one of the nation's top marine biologists, Steve Palumbi, director of Stanford University's Hopkins Marine Station in Pacific Grove.

The vast majority of the world's scientists -- including those at NOAA, NASA, the National Academy of Sciences and the World Meteorological Organization -- say the Earth's temperature is rising because of humans burning fossil fuels like oil and coal. That burning pumps carbon dioxide into the atmosphere and traps heat, similar to a greenhouse. Concentrations of carbon dioxide in the Earth's atmosphere have increased 25 percent since 1960 and are now at the highest levels in at least 800,000 years, according to measurements of air bubbles taken in ancient ice and other methods.

Many of the impacts are already being felt. Since the 1880s, when modern temperature records were first taken, the 10 hottest years have all occurred since 1998. Polar ice has melted, forest fires are burning in the West with increasing frequency, and the ocean has risen 8 inches since 1900 at the Golden Gate Bridge.

But what many people do not realize is that nearly a third of carbon dioxide emitted by humans is dissolved in the oceans. Some of that forms carbonic acid, which makes the ocean more corrosive.

Over the past 200 years, the ocean's acidity has risen by roughly 30 percent. At the present rate, it is on track to rise by 70 percent by 2050 from preindustrial levels.

More acidic water can harm oysters, clams, corals and other species that have calcium carbonate shells. Generally speaking, increasing the acidity by 50 percent from current levels is enough to kill some marine species, tests in labs have shown.

The new research on the marine snails does not show that increasingly acidic water is killing all of them, particularly older snails. But it is causing their shells to dissolve, which can make them more vulnerable to disease, slow their ability to evade predators and reduce their reproductive rates, the researchers said.

Some of the corrosive water near the shore could be a result of other types of pollution, such as runoff from fertilizer and sewage, said Stanford's Palumbi, who was not involved in the NOAA research. But because the study found rates of the snails' shells dissolving in deep water, far from the shore, human-caused carbon dioxide is the prime suspect, he added.

If people reduce emissions of fossil fuels, cutting carbon dioxide levels in the decades ahead, the damage to the oceans can still be limited, he said.

"But if we keep on the emissions profile we have now, by 2100 the oceans will be so harmed it's hard to imagine them coming back from that in anything less than thousands of years," Palumbi said.

"We are in a century of choice," he said. "We can choose the way we want it to go."